Raman light scattering has been successfully used in critical care situations to continuously monitor a patient's respiratory gases. This technique is based on the effect which occurs when monochromatic light interacts with vibrational/rotational modes of gas molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering gas molecules. If the incident light photon loses energy in the collision, it is reemitted as scattered light with lower energy and consequently lower frequency than the incident photon. In a similar manner, if the incident photon gains energy in the collision, it is reemitted as scattered light with higher energy and higher frequency than the incident photon. Since these energy shifts are species-specific, analysis of the various frequency components present in the Raman scattering spectrum of a sample provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components, i.e., Raman spectral lines, provides quantification of the gases present, providing suitable calibrations have been made. In this manner, Raman light scattering can be employed to determine the identity and quantity of various respiratory and anesthetic gases present in a patient's breath in operating room and intensive care situations.
Systems developed for analysis of gases in critical care situations utilizing Raman scattering typically employ gas cells which contain a sample of the patient's respiratory gas to be analyzed. One such system is described in U.S. Pat. No. 4,784,486, entitled "MULTI-CHANNEL MOLECULAR GAS ANALYSIS BY LASER-ACTIVATED RAMAN LIGHT SCATTERING", issued to Van Wagenen et al. The gas cell is located either within the resonant cavity of a laser or outside the cavity. In an intracavity system, such as that described by Van Wagenen, a laser beam is directed through a resonant cavity such that it intercepts a respiratory gas sample within a gas cell. An end mirror located at one end of the resonant cavity redirects light incident from a plasma discharge tube back through the resonant cavity, where it again passes through the gas cell and back into the plasma discharge tube. A Brewster prism may be mounted near the end mirror to select the desired wavelength and polarization state of the lasing light. The end mirror and Brewster prism are both mounted on one or more plates of an alignment assembly. Raman scattered light from the gas analysis region within the gas cell is collected by collection optics and directed through one or more interference filters or other means of wavelength discrimination. The collection optics and interference filters and possibly focusing optics in turn transmit the Raman scattered light to appropriate detectors for quantifying each specific Raman signal and thus each specific gas comprising the respiratory gas sample.
Intracavity systems possess the advantage that they achieve a much greater Raman scattering intensity than systems in which the Raman scattering occurs outside of the laser resonant cavity. This greater intensity is a result of the fact that a laser beam transiting an intracavity arrangement propagates through the gas sample a great many times, with a correspondingly higher time-integrated intensity of Raman scattered light from the gas sample. In contrast, an external arrangement of the gas cell allows the laser beam only one pass through the gas sample. While intracavity systems benefit from a much greater Raman signal strength than do systems having the gas cell located outside the resonant cavity, the resonator optics must be positioned with extreme accuracy for this advantage to be realized, since the multiple reflection of the laser beam within the cavity magnifies any misalignment of the cavity end mirror. Consequently, the cavity end mirror, the Brewster prism (if present), and the central axis of the laser plasma tube must all be aligned almost perfectly with respect to each other at all times during operation of the gas analysis system. The alignment of the end mirror and associated prism is controlled by adjustment mechanisms on the alignment assembly on which the mirror is mounted.
In the intracavity gas cell systems discussed above, windows are commonly provided on either end of the gas cell to protect surrounding optical elements and filters from contaminants which may be present in the gas sample. The windows further serve to confine the gas sample within the gas cell, thereby minimizing the volume of the sample and thus improving the detector's response time. In some systems, the gas cell windows are oriented at Brewster's angle to select and improve the transmission of a particular polarization state of light passing through the sample. In this manner, optical losses in the laser beam which passes through the cell are minimized. However, the gas sample, in combination with particulates often carried with the sample, may contaminate the cell windows and degrade the performance of the system. This contamination may result in undesirable light absorption and/or scattering with a consequent decrease in the laser power circulating through the sample cell. If untreated, this contamination will eventually cause the system to cease to function properly.
The problem of window and cavity optics contamination has been partially solved by providing an air dam around the optics of the laser system to shield the optics from contaminated sample. Systems for providing such an air dam are disclosed in U.S. Pat. No. 5,135,304, entitled "GAS ANALYSIS SYSTEM HAVING BUFFER GAS INPUTS TO PROTECT ASSOCIATED OPTICAL ELEMENTS", issued to Miles et al. and U.S. Pat. No. 5,153,671, entitled "GAS ANALYSIS SYSTEM HAVING BUFFER GAS INPUTS TO PROTECT ASSOCIATED OPTICAL ELEMENTS", issued to Miles. In intracavity systems such as those disclosed in U.S. Pat. No. 5,135,304, the sample of gas to be analyzed is injected near the center of the array of detectors. Simultaneously, a buffer gas such as nitrogen or filtered air is injected on the sides of the analyzer cavity. Both gas streams are exhausted at an intermediate point. This system advantageously provides a pure gas sample near the detectors while protecting the optics of the resonant cavity from contamination.
In spite of the advances made in protecting the resonant cavity optics from contamination, individual portions of the resonant cavity, including the end mirrors, gas cell and laser plasma tube, must still occasionally be disassembled and cleaned of contamination, repaired or replaced. At such times, the optical elements are disassembled and repaired or cleaned, then reassembled. The high degree of precision required of the optical alignment of the system, including the alignment of the end mirror, Brewster prism (if present), and plasma discharge tube, renders field repairs difficult. Thus, most repairs are presently made at the factory where the system can be placed on an optical bench or fixture for precision alignment of the components.
Many devices have been created to aid in the alignment of optical elements in general and for alignment of laser system components in particular.
However, most of these devices suffer from one or more of the following disadvantages: 1) complex and hence expensive mechanisms; 2) low accuracy; 3) unstable, i.e., do not hold adjustment well over time; or 4) must be carefully realigned each time the system configuration is altered. For example, U.S. Pat. No. 4,442,524, entitled "MULTIPLE LEVER APPARATUS FOR POSITIONING AN OPTICAL ELEMENT", issued to Reeder et al. discloses a system for finely adjusting the relative angular orientation of a mirror assembly. This system employs a complex set of levers to reduce the amount of adjustment made by a single turn of each adjusting screw. The Reeder system has very little backlash and does not use an extra-fine screw to directly move the mirror plate. This system is expensive to manufacture and appears to be unstable, i.e., would not hold an adjustment during temperature changes and may be sensitive to vibrations. U.S. Pat. No. 4,878,227, entitled "DEVICE FOR A MODULAR POWER LASER", issued to Ackermann et al. discloses a system wherein two plates are supported relative to each other with a "differential" thread adjustment screw arrangement. However, the "differential" thread of Ackermann is a combination of a right hand screw pitch in one plate and a left hand screw pitch in the other plate, connected by an adjustment screw having a right hand thread pitch on one end and a left hand thread pitch on the other end, each end having the same number of threads per inch, n. Thus, the effect of this arrangement is that a single turn of the adjustment screw changes the relative distance between the plates by 2/n inches. This is not suitable for precision adjustments, since the adjustment is too course.
Additional optical adjustment devices are disclosed in: U.S. Pat. No. 4,796,275, entitled "FLOATING MIRROR MOUNT", issued to Koop; U.S. Pat. No. 4,680,771, entitled "MIRROR ADJUSTMENT DEVICE IN LASER OSCILLATOR", issued to Koseki; U.S. Pat. No. 4,672,626, entitled "LASER Oscillator MIRROR ADJUSTMENT APPARATUS", issued to Koseki; U.S. Pat. No. 4,638,486, entitled "ADJUSTMENT DEVICE FOR A REFLECTOR MIRROR OF A LASER RESONATOR", issued to Dost et al.; U.S. Pat. No. 4,515,447, entitled "OPTICAL ADJUSTMENT DEVICE", issued to Weimer et al.; U.S. Pat. No. Re. 31,279, entitled "LASER OPTICAL RESONATOR", issued to Mefferd et al.; U.S. Pat. No. 3,864,029, entitled "LASER MIRROR MOUNT AND WINDOW PROTECTION ASSEMBLY", issued to Mohler; U.S. Pat. No. 3,564,452, entitled "LASER WITH STABLE RESONATOR", issued to Rempel; U.S. Pat. No. 3,484,715, entitled "TEMPERATURE COMPENSATING MOUNTING FOR LASER REFLECTORS", issued to Rempel; U.S. Pat. No. 3,359,812, entitled "ANGLE ADJUSTING MECHANISM FOR OPTICAL ELEMENTS", issued to Everitt; Japan Patent No. 63-70469(A), entitled "ADJUSTING DEVICE FOR RESONATOR MIRROR", issued to Mitsubishi; Japan Patent No. 59-204291(A), entitled "LASER RESONATOR", issued to Nihon.
In general, commonly used optical alignment devices are not suitable for several aspects of a mass produced, field serviceable, robust, complex optical system, such as a Raman Gas Analysis system. Simple set screw mechanisms suffer from backlash between the threads and the securing bolt. This backlash allows the position of the element being adjusted to shift after the adjustment, necessitating the employment of biasing springs or other means to provide a constant pressure on the threads. More complex systems require precision alignment on an optical bench each time the system configuration is disturbed, either intentionally for repairs or unintentionally by environmental factors such as vibration and temperature changes. Thus, there exits a need for optical alignment devices which are robust in that they are easily aligned in the field and will maintain their alignment once fixed. The devices of the present invention satisfy these requirements.